This invention relates to an amplitude limiter circuit for limiting the amplitude of an output signal.
An amplitude limiting circuit is used to limit the amplitude of an output signal within a predetermined range and is referred to as a limiter circuit or a slice circuit, depending on its applications. A circuit schematic of a typical conventional amplitude limiter circuit is shown in FIG. 1. Desired or limiting voltages, e.g. +V.sub.1 and -V.sub.2 are applied to one terminal of each diodes 10 and 12. When the input voltage Vi (for example a triangle waveform in FIG. 2A) applied from a signal source (not shown) through input terminal 2 is within the amplitude limiting range, i.e., -V.sub.2 &lt;V.sub.i &lt;+V.sub.1, both diodes 10 and 12 are OFF to allow the input signal to pass to output terminal 6 through resistor 14. However, diode 10 alone is ON to clamp the output voltage to the predetermined upper voltage V.sub.1 for +V.sub.1 &lt;V.sub.i, and diode 12 along is ON to clamp the output voltage to the -V.sub.2 for V.sub.i &lt;-V.sub.2 (see FIG. 2B).
Now, turning on either diode 10 or 12 causes a current (I.sub.R in FIG. 2C) to flow into the signal source through resistor 14, thereby causing undesirable effects such as distortion upon the signal waveform. Letting stray capacitance of diodes 10 and 12 and resistance of resistor 14 be respectively Cd and R, the time constant TC of the circuit in FIG. 1 is given by the following expression: EQU TC=2R Cd.
Considering load capacitance C.sub.L between the output terminal 6 and ground, EQU TC=R (2Cd+C.sub.L).
This suggests that the use of larger resistance R to reduce the current I.sub.R and, in turn, the loading effect to the signal source, the time constant increases to degrade the frequency response. On the contrary, an attempt to improve the frequency response by using smaller resistance R will result in increasing the loading effect to the signal source.
It is therefore an object of this invention to provide an amplitude limiter circuit with minimum loading effect to the signal source connected to the input terminal and excellent frequency response.
Other objects and advantages will become apparent to those skilled in the art upon a reading of the following description when taken in conjunction with the drawings.